GUN CONSTRUCTION In the early days of ordnance, gunnery was in the realm of the magic arts. It was not until the sixteenth century that it was treated in a scientific spirit by Niccolo Tartaglia. Later Ben jamin Robins advanced the study, but results of his research and experiment were not fully applied until the evolutionary epoch, when gunnery passed into the realm of science.
During the twentieth century much attention has been de voted not only to the structure of the gun, but to its interior design to obtain improved ballistics, so that higher efficiency and greater accuracy might be achieved. The nature and form of rifling, the size and form of chamber, the character (particularly in regard to regularity in combustion and freedom from age de terioration), weight and form (size) of the propellent are matters for research and experiment in which progress still continues to be made. Four important general requirements of a gun are (I) high muzzle velocity; (2) accuracy; (3) good life; (4) rapidity of fire. The relative importance of these depends on the special conditions for which the gun is required. All affect the pros pects of hard and frequent hitting which are the main desiderata.
This depends on the pressure de veloped during the projectile's passage down the bore and the distance through which this pressure acts, and the frictional resistance encountered by the projectile. Limitations in maxi mum pressure are decided by the strength of the steel of which the gun is constructed, and to some extent by the pressure that can be withstood by the weakest type of shell that the gun is required to fire, such as large capacity high explosive shell. In modern guns this maximum pressure rarely exceeds 22 tons per square inch. Limitations are imposed on the length of the gun by the circumstances of its use, such as considerations of weight and the stresses produced on the mounting or structure which carries it; by requirements in regard to the position of the centre of gravity; and by the amount of deflection or gravitational droop which is permissible in the unsupported chase of the gun.
Modern guns are generally between 45 and 55 calibres in length, that is, the length of the bore and chamber of a so calibre 6-inch gun is 25 feet. (In some countries the overall length of the gun is referred to.) Accuracy.—This depends upon the uniformity of pressure and of friction for successive rounds and on the stiffness of the gun structure. Both pressure and friction are affected by the wear which they occasion in the bore and chamber. The wear increases the size of the chamber by allowing the projectile to seat further forward and affects the frictional resistance to the driving band of the projectile and possibly the efficiency of the ing, which also necessitates provision for extinguishing all smouldering residue.
The designer being given a required muzzle velocity and length of gun and a limiting maximum pres sure, is confronted with the problem of deciding the most suitable size of chamber. A gun may be viewed as a single stroke internal combustion engine, the piston of which, represented by the pro jectile, is blown through the open end of the cylinder at each stroke, after being rotated by the rifled walls of the cylinder. The latter engage in the copper driving band, which also serves as a piston packing ring and seals the gases. The same muzzle velocity and maximum pressure can be obtained in the same length of gun by chambers of different capacity, provided suitable varia tions are made in the weight and form of the charge. 'Different sizes of chambers are ' favoured by different nations; the point is important as it has a direct bearing on the accuracy or regular ity of the gun. By means of internal ballistic calculations based on theory and supported by data obtained from experiments with gases in closed vessels, pressure curves can be produced for different sizes of chamber and weight and form of charge. The form of charge or size of the elements of which it is composed, affects through the surface exposed, the rate of burning.